BIOLOGY 5 Marks Questions

Cell Size and Metabolic Rate: Size of the cells vary considerably as we have already seen. But in most of the cells, size ranges between 3 to 30 microns.
The group of smaller cells have a higher metabolic rate than a larger cell because of the following factors:
i. Nucleocytoplasmic ratio: We know that that nucleus controls all the metabolic activities of the cell including growth, development, etc. However the nucleus cannot extend its control over an indefinitely large area and without the control of the nucleus, the cell cannot function. If the size increases, the metabolic rate decreases.
ii. Surface area of the cell: The substances required for metabolism pass into the cell through the cell membrane, for example, oxygen. If the size of the cell is big, all the areas of the cell may not get the required amount of oxygen. Hence metabolism shall decrease. It is on this account that the group of smaller cells have a higher metabolic rate than the bigger single cell of the same size.

The basic structural organisation of a typical cell is as follows
i. Nucleus, the central part and brain of the cell, which is spherical in shape. Its number can be one or more per cell. It is denser than the surrounding cytoplasm.
ii. Cytoplasm, a semi-fluid matrix that occupies the volume of the cell. It is mainly composed of water with free-floating molecules. Inside the cytoplasm, all cellular activities like a gaseous exchange, elimination of wastes, hereditary mechanisms, etc., occur.
iii. The outer membrane, the boundary of the cell, which provides protection to the cell and controls the exchange of ions, molecules and other components in and out of the cell.
The outer membrane of a cell contains cell wall (only in plant cells) and plasma membrane.

Electron Transport System (ETS): The metabolic pathway by which the electrons passes from one carrier to another is known as the electron transport system. It is operative in the inner mitochondrial membrane of mitochondria. The electrons from NADH produced in the mitochondrial matrix during the citric acid cycle are oxidised by an NADH dehydrogenase (Complex I). Electrons are then transferred to Ubiquinone that receives reducing equivalents via FADH, {generated during oxidation of succinate) by the activity of Succinic dehydrogenase (Complex II) in TCA. Reduced ubiquinone is oxidised with the transfer of electrons to cytochrome V via Cytochrome V complex (complex III). Cytochrome V acts carrier for transfer of electrons between complex III and complex IV. Complex IV refers to cytochrome c oxidase complex having cytochromes a and 3 and two copper centres.

This figure shows the major pathways of anaerobic respiration or fermentation. Glycolysis is shown on the lefthand side, while further processing of pyruvic acid is shown on the right-hand side.
In animal cells also, like muscles during exercise, when oxygen is inadequate for cellular respiration pyruvic acid is reduced to lactic acid by lactate dehydrogenase. In certain organisms, pyruvic acid is processed to produce ethanol and carbon dioxide. Some amount of energy is released in both cases.
Two applications of anaerobic respiration are as follows:
i. In making fluffy cakes and bread anaerobically reproducing fungi yeast is used.
ii. In making curd lactobacillus is added as inoculum.

Homologous Chromosomes:These are pairs of similar chromosomes having corresponding genes governing the same set of traits.
During the heterotypic division of meiosis in leptotene, chromosomes are thread shaped and coiled. During zygotene, the homologous chromosomes start pairing. Here morphologically and genetically chromosomes similar are called homologous chromosomes. In pachytenes, the chromosomes show thickening and shortening. Diplotene is marked by the cessation of attraction force between two homologous chromosomes. Uncoiling of homologous chromosome tends to separate them from each other but remains attached at chiasmata. During diakinesis, the separation of homologous chromosomes is complete.
Exchange of parts between chromatids of homologous chromosomes may occur.
During Anaphase I, the centromere of homologous compounds of bivalents repel each other. After separation of centromeres, the homologous chromosomes begin to move apart towards the spindle. In telophase I, the chromosomes reach poles and become shortened. The two cells have a reduced number of chromosome and then second meiosis begins.

Cell division is significant in the following ways
i. Cell multiplication: Cell division is a means of cell multiplication or the formation of new cells from pre-existing cells.
ii. Continuity: It maintains continuity of living matter generation after generation.
iii. Multicellular organisms: The body of a multicellular organism is formed of innumerable cells. They are formed by repeated divisions of a single cell or zygote. As the number of cells increases, many of them begin to differentiate, form tissues and organisms.
iv. Cell size: Cell division helps in the maintenance of a particular cell size which is essential for efficiency and control of cell activities.
v. Genetic similarity: The common type of cell division or mitosis maintains the genetic similarity of all the cells in an individual despite being different, i.e., structurally and functionally.